Regenerative thermal oxidizers are conventionally used for destroying volatile organic compounds (VOCs) in high flow, low concentration emissions from industrial and power plants. Such oxidizers typically require high oxidation temperatures in order to achieve high VOC destruction. To achieve high heat recovery efficiency, the xe2x80x9cdirtyxe2x80x9d process gas that is to be treated is preheated before oxidation. A heat exchanger column is typically provided to preheat these gases. The column is usually packed with a heat exchange material having good thermal and mechanical stability and sufficient thermal mass. In operation, the process gas is fed through a previously heated heat exchanger column, which, in turn, heats the process gas to a temperature approaching or attaining its VOC oxidation temperature. This pre-heated process gas is then directed into a combustion zone where any incomplete VOC oxidation is usually completed. The treated now xe2x80x9ccleanxe2x80x9d gas is then directed out of the combustion zone and back through the heat exchange column or through a second heat exchange column. As the hot oxidized gas continues through this column, the gas transfers its heat to the heat exchange media in that column, cooling the gas and pre-heating the heat exchange media so that another batch of process gas may be preheated prior to the oxidation treatment. Regenerative thermal oxidizers often have at least two heat exchanger columns that alternately receive process and treated gases. This process is continuously carried out, allowing a large volume of process gas to be efficiently treated.
The performance of a regenerative oxidizer may be optimized by increasing VOC destruction efficiency and by reducing operating and capital costs. The art of increasing VOC destruction efficiency has been addressed in the literature using, for example, means such as improved oxidation systems and purge systems (e.g., entrapment chambers), and three or more heat exchangers to handle the untreated volume of gas within the oxidizer during switchover. Operating costs can be reduced by increasing the heat recovery efficiency, and by reducing the pressure drop across the oxidizer. Operating and capital costs may be reduced by properly designing the oxidizer and by selecting appropriate heat transfer packing materials.
An important element of an efficient oxidizer is the valving used to switch the flow of process gas from one heat exchange column to another. Any leakage of untreated process gas through the valve system will decrease the efficiency of the apparatus. In addition, disturbances and fluctuations in the pressure and/or flow in the system can be caused during valve switchover and are undesirable. Valve wear is also problematic, especially in view of the high frequency of valve switching in regenerative thermal oxidizer applications. Frequent valve repair or replacement is obviously undesirable.
One conventional two-column design uses a pair of poppet valves, one associated with a first heat exchange column, and one with a second heat exchange column. Although poppet valves exhibit quick actuation, as the valves are being switched during a cycle, leakage of untreated process gas across the valves inevitably occurs. For example, in a two-chamber oxidizer during a cycle, there is a point in time where both the inlet valve(s) and the outlet valve(s) are partially open. At this point, there is no resistance to process gas flow, and that flow proceeds directly from the inlet to the outlet without being processed. Since there is also ducting associated with the valving system, the volume of untreated gas both within the poppet valve housing and within the associated ducting represents potential leakage volume. Since leakage of untreated process gas across the valves leaves allows the gas to be exhausted from the device untreated, such leakage which will substantially reduce the destruction efficiency of the apparatus. In addition, conventional valve designs result in a pressure surge during switchover, which exasperates this leakage potential.
Rotary style valves have been used to direct flow within regenerative thermal and catalytic oxidizers for the past ten years. These valves either move continuously or in a digital (stop/start) manner. In order to provide good sealing, mechanisms have been employed to keep constant force between the stationary components of the valve and the rotating components of the valve. These mechanisms include springs, air diaphragms and cylinders. However, excessive wear on various components of the valve often results.
It would therefore be desirable to provide a valve and valve system, particularly for use in a regenerative thermal oxidizer, and a regenerative thermal oxidizer having such a valve and system, that ensures proper sealing and reduces or eliminates wear.
It also would be desirable to provide and valve and valve system wherein the sealing pressure can be precisely controlled.
The problems of the prior art have been overcome by the present invention, which provides a lift system for a switching valve, the switching valve, and a regenerative thermal oxidizer including the lift system and switching valve. The valve of the present invention exhibits excellent sealing characteristics and minimizes wear. The lift system assists the valve in rotating with minimal friction and providing a tight seal when it is stationary. In a preferred embodiment, the sealing force of the valve against the valve seat is reduced during switching to reduce the contact pressure between the moving components and the stationary components, thus resulting in less required torque to move the valve.
For regenerative thermal oxidizer applications, the valve preferably has a seal plate that defines two chambers, each chamber being a flow port that leads to one of two regenerative beds of the oxidizer. The valve also includes a switching flow distributor that provides alternate channeling of the inlet or outlet process gas to each half of the seal plate. The valve operates between two modes: a stationary mode; and a valve movement mode. In the stationary mode, a tight gas seal is used to minimize or prevent process gas leakage. In accordance with the present invention, during valve movement, the sealing pressure is reduced or eliminated, or a counter-pressure or counter-force is applied, to facilitate valve movement and reduce or eliminate wear. The amount of sealing pressure used can be precisely controlled depending upon process characteristics so as to seal the valve efficiently.